Without limiting the scope of the invention, its background is described in connection with multimode interference couplers and photonic crystal waveguides. Introducing line defects into a photonic crystal lattice permits an electromagnetic wave having a frequency within the bandgap of the structure to be guided through the photonic crystal. The line defects resemble waveguides, and may be formed by either adding or removing dielectric material to a certain row or column along one of the directions of the photonic crystal lattice. Thus, photonic crystal waveguides can be used as an optical “wire” to guide an optical signal between different points, or devices, within an optical integrated circuit.
Photonic crystal waveguides (PCW) with low group velocity have been demonstrated recently to replace conventional optical switches and modulators [1-4], where the size of the active region is considerably reduced via slow light effect [5,6]. They typically consist of a periodic array of air holes on a dielectric substrate whose optical properties are modified by an external physical signal. One of the most efficient tuning methods may be based on the application of electro-optical material. It is well known that the unique properties of photonic crystals can be exploited to enhance the nonlinear effect drastically and thus a small attainable change in the refractive index can induce applicable optical response [7].
For example, U.S. Pat. No. 6,782,169 discloses a coupler having a dielectric mirror or Gaussian mirror to couple optical signals from a planar waveguide to a PCW. Although the mirrored coupler provides a coupling efficiency of greater than 80%, the coupler does not permit the optical path of the planer waveguide to be longitudinally aligned with the optical path of the PCW. In addition, the actual coupling efficiency of the coupler is dependant on the manufacturing quality of the mirror, which necessarily increases the cost and complexity of the coupler.
As disclosed in U.S. Pat. No. 5,889,906, multimode interference couplers of varying shapes have been used. The width of the wide end of these couplers is about 15 μm and the length of these couplers range between 200 μm and 600 μm. Another example of tapered couplers is disclosed in United States Patent Application Publication 2002/0159703. The width of the wide end of these couplers range between 10 μm and 50 μm, and the length of these couplers range between 150 μm to 500+ μm. These couplers require a taper length of more than several hundred of microns to reduce the propagation loss due to the requirement of adiabatic tapering.
The inventors previously disclosed that a simple MMI coupling structure between a channel waveguide and a PCW with a center trench can achieve a coupling efficiency of 70%. See X. Chen, L. GU, W. Jiang, R. Chen, “Active transmission control based on photonic-crystal MOS capacitor,” Proc. of SPIE, vol. 6480, 64800W (2007), which is hereby incorporated by reference in its entirety. The MMI section interfaces with the slot PCW at the edge of the period that gives a termination parameter of τ=0.5. The width of the coupler is equal to the defect width of the slot PCW and the length of the coupler is approximately 1.25 times the width of the coupler.
As a result, there is a need for a MMI coupling structure that is small, does not require complicated structures or fabrication, yet provides a coupling efficiency that exceeds 90%.